GolfScript (4343 42 chars)
' /\ /__\ '4/){.+\.{2$[2$.@\++]*}%\{.+}%+\}3*;n* Output:
/\ /__\ /\ /\ /__\/__\ /\ /\ /__\ /__\ /\ /\ /\ /\ /__\/__\/__\/__\ /\ /\ /__\ /__\ /\ /\ /\ /\ /__\/__\ /__\/__\ /\ /\ /\ /\ /__\ /__\ /__\ /__\ /\ /\ /\ /\ /\ /\ /\ /\ /__\/__\/__\/__\/__\/__\/__\/__\ Change the "3" to a larger number for a larger triangle.